Wind turbine rotors may turn out to be unbalanced at the installation of the wind turbine. The unbalance may be caused by differences in blade weight, more precisely by blade root bending moment caused by gravity, by differences in adjustment of the blade pitch setting on the pitch mechanism of the rotor hub, or by aerodynamic differences from one blade to the other. Unbalances due to the blade root bending moment can arise i.e. from water penetrating the blade thus increasing its mass, or from loose parts inside the blade which move, e.g. outwards during rotor revolution. Aerodynamic differences between blades may be caused, e.g., if the angles of attack differ from one blade to another.
While unbalances resulting from differences in the blade root bending moment or in the adjustment of the blade pitch setting manifest themselves in a periodic transverse oscillation relative to the rotor axis with the same frequency as the rotation of the rotor (the so-called 1p frequency), aerodynamic unbalances manifest themselves in a periodic force having a frequency which corresponds to three times the rotor frequency for a usual three bladed rotor blade. This frequency is called 3p-frequency. When operating with an unbalanced rotor, a wind turbine will experience higher structural loads compared to a wind turbine operating with a balanced rotor, which reduces the lifetime of the rotor.
In order to reduce the load acting on wind turbine rotor blades, EP 1 666 723 A1 describes a blade pitch angle control device which calculates individual blade pitch angles for different blades on the basis of the measured wind speed, the air density, the rotor's azimuth angle and the wind turbine's power generator output. The individual blade pitch angle control signals are added to a common pitch angle signal so as to build up the control signal delivered to the blade's pitch mechanisms. In an alternative embodiment individual load measuring units are provided for each rotor blade instead of wind speed detecting units, air density detecting units and wind power generator output detecting units. In any case, a number of detecting units need to be provided for gathering the necessary information for the control unit to calculate the individual blade pitch angle control signals.
EP 1 544 458 A2 describes a pitch angle control for a wind turbine rotor. The pitch angle is set on the basis of the detected load on a part of the wind turbine unit. The loads may be detected on the basis of the measured wind speed or by a direct measurement of the mechanical load acting on a part of the rotor, in particular on the rotor blades.
A method of monitoring the conditions of rotor blades is described in P. Caselitz and J. Giebhardt “Rotor condition monitoring for improved operational safety of offshore wind energy converters”, Journal of Solar Energy Engineering, May 2005, Vol. 127, Issue 2, Pages 253 to 261. The condition of the rotor is determined on the basis of calculating and monitoring the 1p-oscillations of the nacelle. Based on the power output and the amplitude of the 1p-oscillation the condition of the rotor is classified as “okay” or “fault”.